KVM: arm/arm64: vITS: Add a helper to update the affinity of an LPI

In order to help integrating the vITS code with GICv4, let's add
a new helper that deals with updating the affinity of an LPI,
which will later be augmented with super duper extra GICv4
goodness.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>